1. Field of the Invention
The present invention relates to a gas laser device which controls to make constant a time between the input of an outer trigger and the emission of laser light.
2. Description of the Related Art
As a light source for a reduced projection exposure device (hereinafter referred to as a xe2x80x9cstepperxe2x80x9d) for production of semiconductor devices, a gas laser device is used, and attention is especially being given to the use of an excimer laser among others.
FIG. 9 is a diagram showing the structure of an excimer laser device 10.
The excimer laser device 10 is comprised of a laser chamber 11 which has therein discharge electrodes for causing an electric discharge therebetween to excite laser gas so to output laser light, a pulse power source 12 which applies a high frequency voltage to the discharge electrodes, a charger 13 which charges the pulse power source 12, an output mirror 14 which resonates the laser light, apertures 15, 16 which determine a shape of the laser light, and a band narrowing module 17 for narrowing a spectral line width of the laser light.
FIG. 10 is a diagram showing an example of a circuit and peripheral components used for the pulse power source 12. Generally, a magnet compression circuit is used for the pulse power source 12. A three-stage magnet compression circuit is used for the pulse power source 12 shown in FIG. 10.
In the pulse power source 12 shown in FIG. 10, a charging capacitor C0 is connected to the charger 13. An assist coil L0, a semiconductor switch SW and a transfer capacitor C1 are connected in parallel to the charging capacitor C0. A saturable reactor SL1 and a transfer capacitor C2 are connected in parallel to the transfer capacitor C1. A saturable reactor SL2 and a transfer capacitor C3 are connected in parallel to the transfer capacitor C2. A saturable reactor SL3 and a peaking capacitor Cp are connected in parallel to the transfer capacitor C3. Discharge electrodes 21 are connected in parallel to the peaking capacitor Cp.
Energy instruction value E required for each pulse is input to a voltage instruction value arithmetic section 22. In the voltage instruction value arithmetic section 22, charge voltage Vc of the charging capacitor C0 is calculated according to the energy instruction value E, and charge voltage instruction value V0 is output to the charger 13. The charging capacitor C0 is recharged according to the charge voltage instruction value V0.
When a trigger (hereinafter referred to as the xe2x80x9couter triggerxe2x80x9d) TR to be output from a stepper is input to the semiconductor switch SW, the semiconductor switch SW is turned on, and electric charges recharged into the charging capacitor C0 are transferred to the transfer capacitor C1. At this time, when a value obtained by integrating a voltage, which is applied to the saturable reactor SL1, with respect to time reaches a prescribed level, the saturable reactor SL1 is magnetically saturated, and inductance rapidly becomes small. Then, the transfer of electric charges from the front-stage transfer capacitor C1 to the back-stage transfer capacitor C2 is started. Thus, each saturable reactor SLn functions as the magnetic switch which is turned on by magnetic saturation.
Similarly, electric charges are sequentially transferred from the front-stage transfer capacitor Cn to the back-stage transfer capacitor Cn+1 and finally to the final peaking capacitor Cp by the switching function of the respective saturable reactors SLn. The voltage between the discharge electrodes 21 rises along with a voltage increase of the peaking capacitor Cp, and when the voltage between the discharge electrodes 21 reaches a prescribed value, the laser gas between the discharge electrodes 21 is produced an electrical breakdown, and the electric discharge is started. The laser gas is excited by the electric discharge, and the laser light is emitted.
Because it is configured in such a way that the inductance becomes smaller as the process advances from the front-stage saturable reactor SLn to the back-stage saturable reactor SLn+1, the pulse compression is effected so that the peak value of electric current passing through the circuit of each step increases sequentially, and a span of electrifying time becomes narrow. Therefore, a powerful discharge can be obtained between the discharge electrodes 21 in a short time.
The excimer laser device 10 is controlled as described below.
A semiconductor substrate is placed on a stage on the part of the stepper. The outer trigger TR is output from the stepper side to the excimer laser device 10 so to emit the laser light in synchronization with the operation of the stage. In order to perform exposure to light with high accuracy, the excimer laser device 10 must keep constant time Tt between the input of the outer trigger TR and the emission of the laser light. The time Tt will be referred to as total time Tt below.
The total time Tt includes a total value Td of time Tdn until each saturable reactor SLn turns on and delay time Ts peculiar to an LC circuit of the magnet compression circuit. The respective times Td, Ts are referred to as delay times Td, Ts below. The delay time Ts is normally constant.
The delay time Tdn of the saturable reactor SLn is determined by the designs of a magnetic characteristic, a sectional area, a number of turns, and the like of the saturable reactor SLn. The delay time Tdn of the saturable reactor SLn which has such designs determined depends on a time integral value of the voltage applied to the saturable reactor SLn. Normally, the time integral value of the voltage is constant. Specifically, when the voltage applied to the saturable reactor SLn is low, the time to turn on the saturable reactor becomes long, and when the voltage applied to the saturable reactor SLn is high, the time to turn it on becomes short. As shown in FIG. 11, the time integral value of the applied voltage is indicated by an area surrounded by a time axis and a voltage waveform. The time integral value of the voltage will be used as a voltage and time product below. And, the voltage and time products of the respective saturable reactors SLn are assumed to be the voltage and time product of all saturable reactors SL.
The voltage applied to all saturable reactors SL is replaced with a voltage Vc of the charging capacitor C0. Therefore, the delay time Td of all saturable reactors SL varies according to a variation in the voltage Vc of the charging capacitor C0, and the total time Tt varies. Such variations are referred to as jitter.
Technologies for remedying the jitter problem are disclosed in Japanese Patent Application Laid-Open No. 11-289119 (hereinafter referred to as xe2x80x9cPublication 1xe2x80x9d) and U.S. Pat. No. 6,016,325 (hereinafter referred to as xe2x80x9cPublication 2xe2x80x9d). In Publication 1 and Publication 2, time Tc for compensating delay time Td+Ts is determined to make the total time Tt constant. The time Tc will be referred to as the compensation time Tc below.
The voltage Vc of the charging capacitor C0 is determined by an input charge voltage instruction value V0. Therefore, the charge voltage instruction value V0 and the delay time Td are mutually associated in Publication 1, and according to this associated relationship, the compensation time Tc corresponding to the charge voltage instruction value V0 is previously determined so that the total time Tt becomes constant. And, when the charge voltage instruction value V0 is input for each pulse, corresponding compensation time Tc is determined, so that the total time Tt becomes constant.
In Publication 2, not each charge voltage instruction value V0 but actual voltage Vc of the charging capacitor C0 is previously associated with the compensation time Tc.
But, the charge voltage instruction value V0 and the associated relationship of the voltage Vc of the charging capacitor C0 and the delay time Td are not always constant. For example, this relationship varies depending on a change in temperature of the charging capacitor C0. As a result, correct compensation time Tc cannot be obtained.
Therefore, according to the technologies of Publications 1 and 2, it is necessary to operate to change the charge voltage instruction value V0 and the relationship between the voltage Vc of the charging capacitor C0 and the compensation time Tc according to a temperature change. But, such processing is not realistic because a complex process is required.
According to the technologies of Publications 1 and 2, when the charging capacitor C0 is in an isothermal state, the total time Tt is constant, and the semiconductor substrate can be exposed to light accurately, but they have a disadvantage that when the charging capacitor C0 has a temperature change, the total time Tt varies, and the semiconductor substrate cannot be exposed to light accurately.
Besides, the voltage and time product varies depending on a change in temperature of each saturable reactor SLn, and time for pulse compression varies depending on a change in temperature of each transfer capacitor Cn. The delay time Td is also variable depending on such factors. Therefore, it is necessary to set the compensation time Tc according to the state in consideration of the influence of a change in temperature.
The present invention was achieved under the above circumstances, and it is an object of the present invention to make it possible to perform the exposure of a semiconductor substrate with higher accuracy by keeping constant the total time at all times between the entry of the outer trigger and the emission of the laser light even when any component of the pulse power source has a temperature change.
A first aspect of the invention is directed to a gas laser device, comprising:
an electric charge transfer circuit which has a plurality of transfer capacitors and at least one saturable reactor, uses magnetic saturation of the saturable reactor to sequentially transfer an electric charge from a front-stage transfer capacitor to a back-stage transfer capacitor to cause an electric discharge across discharge electrodes which are connected to a final-stage transfer capacitor so to emit pulse laser light;
a charging capacitor which accumulates electric charges to be transferred to a first-stage transfer capacitor among the transfer capacitors;
electric charge amount measuring means which measures an amount of electric charges accumulated in the charging capacitor;
compensation trigger output means which determines a compensation time corresponding to the electric charge amount to make constant a time between the input of an outer trigger and the emission of light across the discharge electrodes for each pulse and outputs a compensation trigger after the lapse of the compensation time from the input of the outer trigger; and
a switch which electrically connects the charging capacitor and the first-stage transfer capacitor according to the input of the compensation trigger.
A second aspect of the invention relates to the first aspect of the invention, wherein the compensation trigger output means previously stores a relationship between the compensation time and the inverse of the electric charge amount and determines the compensation time according to the stored relationship.
The first and second aspects of the present invention will be described with reference to FIG. 1 to FIG. 4.
A charging current flows from the charger 13 to the charging capacitor C0 prior to each pulse. A current sensor (electric charge amount measuring means) 32 measures the charging current, and an integrator 37 calculates electric charge amount Qcn accumulated in the charging capacitor C0 according to the time integral value.
As shown in FIG. 3, the electric charge amount Qcn and the outer trigger TR are input to a controller (compensation trigger output means) 31. When the electric charge amount Qcn is input, the inverse b/(aQcn) (a is the inverse of the capacity of the charging capacitor C0, b is a proportional coefficient) of the electric charge amount is calculated. The controller 31 is determined to have a compensation characteristic as shown in FIG. 2(b). This compensation characteristic is indicated as a proportional relationship between compensation time Tc and variable b/(aQc). When the outer trigger TR is input, the variable b/(aQc) of a ramp wave generator starts to change. The variable b/(aQc) decreases with a lapse of time, and when it matches the inverse b/(aQcn) of the calculated electric charge amount, a compensation trigger TRL is output. This time is compensation time Tc.
When the compensation trigger TRL is input to the semiconductor switch SW, the semiconductor switch SW is turned on, and the electric charges accumulated in the charging capacitor C0 are transferred to the transfer capacitor C1. When the saturable reactor SL1 becomes saturated magnetically, the electric charges transferred to the transfer capacitor C1 are transferred to the back-stage transfer capacitor C2. Thus, the electric charges are sequentially transferred from the front-stage transfer capacitor Cn to the back-stage transfer capacitor Cn+1, and an electric discharge is finally performed between the discharge electrodes 21 which are connected in parallel to the peaking capacitor Cp. Then, the laser light is emitted between the discharge electrodes 21.
According to the first and second aspects of the invention, the compensation time Tc is specified according to the compensation characteristic indicated by the proportional relationship between the variable b/(aQc) and the compensation time Tc using the electric charge amount Qc of the charging capacitor C0. The electric charge amount Qc of the charging capacitor C0 is not affected by a temperature change. Therefore, the compensation characteristic determined according to the electric charge amount is constant, so that the total time Tt can be kept constant even if the charging capacitor C0 has a change in temperature, and the semiconductor substrate can be exposed to light with higher accuracy.
Besides, the value b/(aQc) which is proportional to the inverse of the electric charge amount Qc is determined as a variable, and the variable b/(aQc) and the compensation time Tc can be indicated in a proportional relationship. A circuit for processing the compensation characteristic indicated by the proportional relationship can be configured with ease. Therefore, the controller 31 can be configured with ease.
A third aspect of the invention is directed to the first aspect of the invention, wherein:
a delay time between the output of the compensation trigger and the emission of light across the discharge electrodes and the inverse of the electric charge amount are measured for each pulse, and the delay time and the inverse of the electric charge amount are stored for each pulse; and
the compensation trigger output means determines a relationship between the delay time and the inverse of the electric charge amount on the basis of two or more stored delay times and the inverse of the electric charge amount corresponding to the delay times, and determines the relationship between the compensation time and the inverse of the electric charge amount for each pulse on the basis of the determined relationship between the delay time and the inverse of the electric charge amount.
The third aspect of the invention will be described with reference to FIG. 5 and FIG. 6.
The inverse b/(aQcn) of the electric charge amount and the delay time Tdn are measured for each pulse and stored in a memory 34. For example, the inverses b/(aQc1), b/(aQc2) of the electric charge amounts of the last pulse and the last but one pulse and delay times Td1, Td2 are read from the memory 34 for each pulse, and the latest delay characteristic of the inverse b/(aQc) of the electric charge amount and the delay time Td shown in FIG. 5 is calculated by the expression (1) below.
{b/(aQ/c1)xe2x88x92b/(aQc2)}/{(Td1+Ts)xe2x88x92(Tdd2+Ts)}xe2x80x83xe2x80x83(1)
By the expression (1), the ratio of change of the inverse of the electric charge amount to time in the delay characteristic is determined. This ratio is a gradient of the delay characteristic. And, a value is obtained by multiplying the ratio by xe2x88x921. The obtained value is used as a gradient of the compensation characteristic to determine the latest compensation characteristic. According to the obtained compensation characteristic, the variable b/(aQc) of the ramp wave generator varies, and when it matches the inverse b/(aQcn) of the electric charge amount, the compensation trigger TRL is output. This time to match the inverse becomes the compensation time Tc.
According to the third aspect of the invention, the delay characteristic is determined for each pulse, and the compensation characteristic is determined according to the delay characteristic. Thus, the latest compensation characteristic is always kept determined, so that it is not affected by a change in voltage and time product due to a temperature change of each saturable reactor SLn. Therefore, the total time Tt can be made constant, and the semiconductor substrate can be exposed to light with higher accuracy.
A fourth aspect of the invention is directed to the second aspect of the invention, further comprising:
time difference output means which stores a target time between the input of the outer trigger and the emission of light across the discharge electrodes, measures a time between the input of the outer trigger and the emission of light across the discharge electrodes, determines a time difference between the target time and the measured time, and outputs the time difference to the compensation trigger output means, wherein:
the compensation trigger output means adds the time difference to the compensation time.
A fifth aspect of the invention is directed to the second aspect of the invention, further comprising:
time difference output means which stores a target time between the input of the outer trigger and the emission of light across the discharge electrodes, measures a time between the input of the outer trigger and the emission of light across the discharge electrodes, determines a time difference between the target time and the measured time, multiplies the time difference by a prescribed gain and outputs to the compensation trigger output means; wherein:
the compensation trigger output means adds the time difference, which has the prescribed gain multiplied, to a compensation time of a next pulse.
Sixth to eighth aspects of the invention are directed to the third to fifth aspects of the invention, further comprising:
an electromagnetic coil which detects the emission of light across the discharge electrodes as a change in a magnetic flux density, wherein:
the compensation trigger output means measures the delay time or a time between the input of the outer trigger and the emission of light across the discharge electrodes on the basis of the detected result by the electromagnetic coil.
The fourth to eighth aspects of the invention will be described with reference to FIG. 1, FIG. 8 and FIG. 9.
The controller 31 is previously determined to have a voltage corresponding to a target value of the total time Tt.
Meanwhile, when the outer trigger TR is input to the controller 31 for each pulse, a voltage of the ramp wave generator is started to rise. When the laser light is emitted between the discharge electrodes 21, the electric current flows through an electromagnetic coil 33 which is disposed in the vicinity of the discharge electrodes 21, the emission of the laser light is confirmed by the controller 31, and the voltage of the ramp wave generator for measuring time is stopped increasing. At this time, the voltage of the ramp wave generator for time measurement is sampled and held for measurement of a difference from the target value.
The obtained difference is multiplied by a prescribed gain to have a voltage corresponding to the compensation time Ta, and it is input to the ramp wave generator which generates the variable b/(aQc). Then, for the next pulse, after a lapse of the compensation time Ta from the input of the outer trigger TR, the variable b/(aQc) of the ramp wave generator is started to decrease. And, the compensation trigger TRL is output after a lapse of compensation time Txe2x80x2c.
According to the fourth to eighth aspects of the invention, the factors which cannot be compensated by the first to third aspects of the invention, for example, a change in delay time due to a temperature change of the transfer capacitor Cn can be compensated. Therefore, the total time Tt can be kept constant, and the semiconductor substrate can be exposed to light with higher accuracy.